AC Stopped Working- Compressor or Sensor?

[SpaceFrank]

I was driving my 2008 G8 GT to work one morning, and all of a sudden the AC stopped blowing cold. A buddy of mine has a set of manifold gauges, so we checked the system pressure a few days later. The system is still well charged; I forget the number, but the static pressure with the engine off was definitely over 100 psi. Unfortunately, the high and low side pressures stayed the same even with the AC system turned on full blast, so we know the compressor isn't compressing.

The belt is on and the compressor pulley spins freely with the engine running. I know the AC compressors on these cars are variable-displacement and don't use a traditional clutch, so a clutch failure is out. That leaves either a failure of the variable bits in the compressor, or a pressure sensor failure not allowing the system to come on. I would assume that a sensor failure would give the driver some kind of indication, but I don't get a DIC message or CEL or anything. Just to be sure, I plugged in my HPTuner Pro and found no codes of any kind. The center console display shows A/C ON when you turn it on, it just doesn't blow cold. This leads me to believe that the compressor itself is the problem.

Still, because these compressors are so expensive, I want to rule everything else out first. Judging by the RockAuto parts selection, it appears that the G8 only uses a pressure sensor on the high-pressure side, and there is no low-side sensor. Where does this sensor live? I haven't been able to find it under the hood, and I haven't shelled out for the Helm manuals yet either.

Alternatively, is there anything I'd be able diagnose with my HPTuner? So far all I've done with this thing is record run files and load tune files from Pat G, so I'm still a total noob with it.

Any other possibilities? Common failures seen in this system? My friend has all the equipment and know-how to do a proper purge and recharge, I just don't want to replace the compressor until I know for sure it's the problem.

[Napalm]

So check what temp the computer says the outside air temp is? I don't know for certain if this affects the G8 - but troubleshooting a 10 impala a while ago we found that the car thought the OAT was under 40F and it wouldn't turn on the compressor. no codes - because it thought it was that cold out. Reality - it was 87.

Oddly then - I tried turning on the defroster - and that worked OK. this temp sensor was behind the bumper but easy to get to. was not the IAT for the engine.

So then you want to check the wire harness and make sure the compressor is being activated completely - if so. Then I'd